Chelating resins are used as adsorbents/collectors of heavy metal elements in a solution containing a high-concentration salt, which are difficult to adsorb and collect with ion exchange resins. As the ability of forming a complex with metal elements varies depending on the structure of functional groups, chelating resins having various functional groups such as an iminodiasetic acid (IDA) group, a low-molecular polyamine group, aminophosphoric acid group, an isothionium group, a dithiocarbamic acid group and a glucamine group are commercially available. Among them, chelating resins introducing an IDA group are mainly used, but chelating resins introducing a low-molecular polyamine group are used for removing metals from a solution containing a large amount of alkali metals and alkali earth metals. These chelating resins form a complex with many metals, but the stability constant of a complex formed is significantly low as compared to ethylenediamine tetraacetic acid (EDTA), a typical chelating agent, and there arises a disadvantage that metal removal and collection rates are subject to variation due to interference by contaminant ions if the concentration of those contaminant ions in a treated solution is high.
It is known that in polyaminocarboxylic acid type chelating agents such as IDA and EDTA, the greater the number of repetitions of ethyleneimine is (the greater the chain length is), the higher the stability constant of the complex is (see Non Patent Document 1 and Non Patent Document 2). There is disclosed a chelating resin having an aminocarboxylic acid type functional group with increased chain length. Patent Document 1 discloses a linear diethylenetriamine-N,N,N′,N″-tetraacetic acid type prepared by introducing diethylenetriamine into a base resin at terminal nitrogen, followed by carboxy-methylation. Here, a triethylenetetramine type functional group is described together with the diethylenetriamine type, but in examples in Patent Document 2, there are many obscure matters, the metal adsorption capacity is not significantly high, and effects based on the chain length are not made clear.
It is estimated that by further increasing the chain length of the functional group, the stability constant of the complex is improved, and a plurality of metals can also be adsorbed in one molecule. As an example of immobilization of a long chain functional group, Patent Document 2 discloses a high-molecule type chelating resin having a quaternary ammonium group in which polyethyleneimine is introduced into a tertiary amino group type anion exchange resin. This disclosed example indicates that an epoxy group or aldehyde group is introduced into a tertiary amino group of a weak anion exchange resin, followed by reacting polyethyleneimine with the remaining epoxy group or aldehyde group. It is indicated that the chelating resin thus prepared is effective in removal of metals from an organic solvent. However, this introduction reaction is not a quantitative reaction, and therefore the amount of functional group introduced is considered low, thus making it difficult to prepare a chelating resin having a high metal adsorption capacity.
On the other hand, there is also a problem regarding a form as an adsorbent. The chelating resin is a particulate adsorbent like activated carbon and ion exchange resin, and is used in a wide range of areas such as wastewater treatment and water purification. Water treatment techniques using the particulate adsorbents have been already established, and are believed to be frequently used in the future. However, they must be filled in a specific vessel or tank before being used because of their particulate forms, and may be hard to be applied depending on working conditions and installation environment. That is, for meeting a variety of requests, an adsorbent not only having adsorption characteristics as an adsorbent but also being capable of matching a wide variety of forms including particulate forms is required.
For these problems, a fiber-type chelating adsorbent capable of being easily processed into various forms and meeting a variety of requests has been proposed. Patent Document 3 discloses fibers introducing a chelating functional group by chemical grafting, Patent Documents 4 and 5 disclose fibers introducing a chelating functional group by radical generation/graft polymerization by radiation irradiation, and Patent Document 6 discloses fibers by a method of injecting a low-molecule chelating agent under a high temperature and high pressure. These chelating fibers are believed to have sufficient functions and exhibit quick adsorption characteristics, but have problems in terms of production. Chemical grafting limits the kinds of fibers capable of being grafted and complicates production steps. Radiation grafting has an advantage of being applicable to various fibers as compared to chemical grafting, but requires operations under a specific environment due to handling of radiations, and therefore cannot be considered as a convenient and inexpensive production process. Furthermore, the method of injection/impregnation of a chelating agent has an advantage of allowing various fibers to be used, but is not necessarily a convenient production process because a supercritical fluid such as carbon dioxide is most effective under disclosed conditions and pressure conditions are represented by a very high pressure of 100 atm. (9.8×106 Pa)−250 atm. (2.45×107 Pa).